I want to define a method to make sums between different type numbers:
<T> void add (T one, T two) { T res = one + two; } the above method not work because type erasure convert T into Object and thus the + operator is not defined on Object...
How can do that?
Thanks.
You'll have to use a bounded type parameter:
public <T extends Number> double add (T one, T two) { return one.doubleValue() + two.doubleValue(); } Note that it uses double as return type because that's the primitive numeric type that covers the largest range of values - and one or both parameters could be double too. Note that Number also has BigDecimal and BigInteger as subclasses, which can represent values outside the range of double. If you want to handle those cases correctly, it would make the method a lot more complex (you'd have to start handling different types differenty).
BigDecimal and BigInteger ?
public double add (Number one, Number two). the generics has no effect here
Long covers the largest range of values, while Double covers the largest domain. The range Double covers is non-monotonic (aka discontinuous), meaning that you use exponents to reach larger values than Long will allow, but not all of the intermediate values between any two arbitrary Double values. For example, with a Double you can reach +/- infinity, but almost none of the values between 2^65 and 2^66 (as a random choice). You should think carefully before using this snippet.The "simplest" solution I can think of is this (excuse the casting and auto-boxing/unboxing):
@SuppressWarnings("unchecked") <T> T add(T one, T two) { if (one.getClass() == Integer.class) { // With auto-boxing / unboxing return (T) (Integer) ((Integer) one + (Integer) two); } if (one.getClass() == Long.class) { // Without auto-boxing / unboxing return (T) Long.valueOf(((Long) one).longValue() + ((Long) two).longValue()); } // ... } Add as many types you want to support. Optionally, you could handle null as well...
Look at this discussion on SO: How to add two java.lang.Numbers?
It's about the same as your problem. Either way, you should not use generics for this, why? Simple: because with generics you couldn't add a Float and a Double, which in general you should be able to do!
java.lang.Number, the boiler-plate code would be outside of the method. With generics, it's inside of the method...template <class A> A add (A a, A b) { return (a+b); } int main() { int x =10, y =20; cout <<"The Integer Addition is " << add(x,y); return 0; } 
